A variety of X-ray systems may be employed in airports and other facilities requiring heightened security to scan baggage and other articles or containers for explosives, contraband and/or prohibited or unauthorized subject matter present in the baggage.
Various technologies have been used alone or in combination to increase the likelihood that prohibited items will be detected. For example, dual-energy X-ray detection facilitates determining the atomic number of certain materials and facilitates detecting certain materials that may be hidden within or covered by generally higher density materials. Nuclear Quadruple Resonance (NQR) techniques have been employed to detect specific compounds (e.g., nitrogen based compounds) often present in explosives such as plastic explosives. In general, these techniques are used to supplement various X-ray scanning techniques, such as X-ray Computed Tomography (CT).
X-ray scanning generally includes exposing an article, such as baggage, to electromagnetic radiation and detecting the radiation penetrating the baggage. As the radiation passes through the object, it is absorbed at varying levels by the objects, material and structures within the baggage. Upon exiting the baggage, the radiation impinges on a detector with an intensity related to the attenuation of the radiation caused by various materials comprising the contents of the baggage. By detecting radiation after it has penetrated the baggage, the various absorption characteristics of the material inside the baggage may be obtained.
By applying the radiation at various angles, the values of attenuated radiation incident on the detector can be back-projected to compute an image of a cross-section of the baggage. By repeating the procedure at different locations of the baggage, a number of cross-sections may be generated and correlated to form a 3D image of the baggage. The term “baggage” refers generally to any article having a volume that can contain and/or conceal any of various items or materials and may include, but is not limited to, luggage, packages, bags, boxes, cases, cargo containers or any other vessel having a carrying capacity.
Recent heightened security measures have placed stringent requirements on inspection systems. For example, facilities having baggage security checks, and particularly airports, are moving towards implementing 100% inspection of baggage. In an airport, for example, the shear quantity of passenger baggage places strict throughput constraints on an inspection system. Such constraints may not only prohibit baggage from being manually examined, but may also severely limit operator inspection of images generated by a CT scanner or other X-ray device.
To alleviate throughput bottlenecks that may result from manual inspection, various image processing techniques, recognition algorithms and/or statistical analysis tools have been employed to facilitate automatically analyzing image data generated by an X-ray inspection system to detect prohibited items hidden or contained within an article of baggage.
The United States, the Federal Aviation Administration (FAA) and the Transportation Security Administration (TSA) have established certification standards for automatic detection procedures implemented in an X-ray inspection system. For example, in order for an inspection system to be certified, the system must demonstrate both an acceptable detection rate and an acceptable false alarm rate. However, detection rates and false alarm rates are often in competition. As the detection rate increases, so typically does the false alarm rate. In fact, evidence indicates that incremental increases at higher detection rates may produce disproportionately larger increases in false alarm rates. Previous certification standards generally required an inspection system to perform at a particular detection rate and false alarm rate for items or material meeting or exceeding a specified mass (known as “100% threat weight”).
Recent certification standards may require the same performance but for items having 75% of the threat weight threshold. This stricter measure may cause many conventional detection methods to have unacceptable detection rates and/or false alarm rates. In addition, incremental improvements to conventional detection methods may suffer from a larger and often unacceptable increase in false alarm rates.
Accordingly, many automatic detection methods capable of performing acceptably under previous certification procedures and mandates fail to meet the standards requested by the government and government agencies. The stricter requirements have had a particularly harsh effect on conventional detection methods with respect to a specific class of explosives known as sheet explosives.
Explosives can generally be divided into two main categories; bulk explosives and sheet explosives. Bulk explosives are typically more readily detected by conventional automatic detection methods. For example, bulk explosives tend to have mass distributed more uniformly over three dimensions. In contrast, sheet explosives have a characteristic “sheet-like” appearance and tend to have mass distributed over two dimensions while a third dimension (i.e., a depth or thickness) is generally very small. The characteristically thin dimensions of sheet explosives make detection of such material difficult for conventional automatic detection methods, particularly under the new 75% threat weight requirement.
Accordingly, conventional automatic detection systems may be inadequate to meet requirements established by recent security measures. In addition, improvements to conventional automatic detection methods are likely to incur unacceptable false alarm rates.